A conventionally known evaporator has in the airflow direction two heat exchangers, namely, an entry-side heat exchanger (=leeward side heat exchanger) and an exit-side heat exchanger (=windward side heat exchanger) placed opposite each other, each of which has an upper tank, a lower tank, and a plurality of heat exchange passages connected between both of the tanks, a plurality of heat exchange passages being sectioned into a plurality of paths (heat exchange passage groups). In addition, a plurality of paths includes, according to a passing order of refrigerant, in the entry-side heat exchanger a first path, a second path and a third path and in the exit-side heat exchanger a fourth path, a fifth path and a sixth path.
Such an evaporator is preferable because cooling of air can be compensated for by the two heat exchangers, so that unevenness of temperature distribution can be reduced compared to that in an evaporator having one heat exchanger. However, when the sectional area of heat exchange passages of each path is equal, a region which can cool down venting wind and a region which can not significantly cool down venting wind are generated, causing unevenness of temperature distribution.
On the other hand, in order to reduce the unevenness of temperature distribution, an evaporator in which the number of heat exchange passages in the path where the refrigerant flows upward is set smaller than the number of heat exchange passages in the path where the refrigerant flows downward is proposed (for example, JP2005-83677A).
In order to further reduce the unevenness of temperature distribution, an evaporator is also proposed in which the number of heat exchange passages in the first path is set to be smaller than the number of heat exchange passages in any other paths in the entry-side heat exchanger, and the number of heat exchange passages is gradually increased from the fourth path to the last path (sixth path) in the exit-side heat exchanger (for example, JP2006-242406A).
The conventional evaporator described in JP2005-83677A, however, has the following problem. The number of heat exchange passages in the first path where the refrigerant flows downward is increased, so that when the refrigerant flow volume is small, a region where the refrigerant flow volume is reduced is generated in the back side of the longitudinal direction of the tank of the first path in the entry-side heat exchanger (=leeward side heat exchanger), and a high temperature portion is locally generated in the region where the refrigerant flow volume is reduced.
The conventional evaporator described in JP2006-242406A also has the following problem. The number of heat exchanging passages in the sixth path where the refrigerant flows upward is increased, so that a region where the refrigerant flow volume is reduced is generated in the front side of the longitudinal direction of the tank of the sixth path in the exit-side heat exchanger (=windward side heat exchanger), and a high temperature region is locally generated in the region where the refrigerant flow volume is reduced. Summary of the Invention
The present invention has been made in view of the above problems. An object of the present invention is to provide an evaporator which can equalize temperature distribution in a heat exchanger by minimizing a region having a reduced refrigerant flow volume which causes unevenness of temperature distribution.